Here is a new interesting Google Shopping ads test found by Josh Leibner and posted on Twitter. The ad format shows the product listing ads but they are grouped by store and then you can drill into each store's listing by clicking on the ad.
Here is a screen shot of the test:
Here is what happens when you click on the Amazon listing:
As you can see, Google is showing you products by store based on your query.
This is what I see for the same query, just normal products being listed:
Forum discussion at Twitter.